Economized device control for refrigeration systems

ABSTRACT

Methods and systems for operating a refrigeration unit are provided. The methods and systems include measuring a first characteristic of a refrigeration unit, calculating a compressor middle stage pressure based on the first measured characteristic, determining if a first comparison and a second comparison are satisfied based on the first measured characteristic and the calculated compressor middle stage pressure, and opening an economizer solenoid valve when the first comparison and the second comparison are satisfied.

BACKGROUND

The subject matter disclosed herein generally relates to refrigerationsystems and, more particularly, economized device control forrefrigeration systems.

A particular difficulty of transporting perishable items is that suchitems must be maintained within a temperature range to reduce orprevent, depending on the items, spoilage, or conversely damage fromfreezing. A transport refrigeration unit is used to maintain propertemperatures within a transport cargo space. The transport refrigerationunit can be under the direction of a controller. The controller ensuresthat the transport refrigeration unit maintains a certain environment(e.g., thermal environment) within the transport cargo space.

SUMMARY

According to one embodiment, a method for operating a refrigeration unitis provided. The method includes measuring a first characteristic of arefrigeration unit, calculating a compressor middle stage pressure basedon the first measured characteristic, determining if a first comparisonand a second comparison are satisfied based on the first measuredcharacteristic and the calculated compressor middle stage pressure, andopening an economizer solenoid valve when the first comparison and thesecond comparison are satisfied.

In addition to one or more of the features described above, or as analternative, further embodiments of the method may include that thefirst characteristic comprises a plurality of characteristics.

In addition to one or more of the features described above, or as analternative, further embodiments of the method may include that thefirst characteristic comprises a flash tank pressure, a compressorsuction pressure, and a discharge port pressure.

In addition to one or more of the features described above, or as analternative, further embodiments of the method may include that thecompressor middle stage pressure is calculated based on the compressorsuction pressure and the discharge port pressure asP_(mid)=k₁×(P_(suc)×P_(dis))^(k) ² +k₃, wherein k₁, k₂, k₃ are knownparameters.

In addition to one or more of the features described above, or as analternative, further embodiments of the method may include that thefirst characteristic comprises a flash tank pressure, the firstcomparison comprises comparing the calculated compressor middle stagepressure with a critical pressure modified by a first pressure constant,and the second comparison comprises comparing the flash tank pressurewith the critical pressure.

In addition to one or more of the features described above, or as analternative, further embodiments of the method may include monitoring asecond characteristic, determining if a third comparison or a fourthcomparison are satisfied based on the second characteristic, and closingthe economizer solenoid valve when either the third comparison or thesecond comparison are satisfied.

In addition to one or more of the features described above, or as analternative, further embodiments of the method may include that thesecond characteristic is at least one of (i) a time that a flash tankpressure exceeds a critical pressure modified by a first pressureconstant and (ii) the flash tank pressure.

In addition to one or more of the features described above, or as analternative, further embodiments of the method may include that thethird comparison is satisfied if the time that the flash tank pressureexceeds the critical pressure modified by the first pressure constantexceeds a predetermined time.

In addition to one or more of the features described above, or as analternative, further embodiments of the method may include that thefourth comparison is satisfied if the flash tank pressure exceeds thecritical pressure modified by a second pressure constant.

In addition to one or more of the features described above, or as analternative, further embodiments of the method may include, afterclosing the economizer solenoid valve, determining if the economizersolenoid valve should be opened again.

According to another embodiment, a refrigeration unit is provided. Therefrigeration unit includes at least one sensor configured to measure afirst characteristic of the refrigeration unit, a controller configuredto calculate a compressor middle stage pressure based on the firstcharacteristic the controller further configured to determine if a firstcomparison and a second comparison is satisfied based on the firstcharacteristic and the calculated compressor middle stage pressure, andan economizer solenoid valve configured to open when the firstcomparison and the second comparison are satisfied.

In addition to one or more of the features described above, or as analternative, further embodiments of the refrigeration unit may includethat the first characteristic comprises a plurality of characteristics.

In addition to one or more of the features described above, or as analternative, further embodiments of the refrigeration unit may include aflash tank, wherein the first characteristic comprises a flash tankpressure, a compressor suction pressure, and a discharge port pressure.

In addition to one or more of the features described above, or as analternative, further embodiments of the refrigeration unit may includethat the compressor middle stage pressure is calculated based on thecompressor suction pressure and the discharge port pressure asP_(mid)=k₁×(P_(suc)×P_(dis))^(k) ² +k₃, wherein k₁, k₂, k₃ are knownparameters.

In addition to one or more of the features described above, or as analternative, further embodiments of the refrigeration unit may include aflash tank, wherein the first characteristic comprises a flash tankpressure, the first comparison comprises comparing the calculatedcompressor middle stage pressure with a critical pressure modified by afirst pressure constant, and the second comparison comprises comparingthe flash tank pressure with the critical pressure.

In addition to one or more of the features described above, or as analternative, further embodiments of the refrigeration unit may includethat the controller is configured to determine if a third comparison ora fourth comparison are satisfied and wherein the economizer solenoidvalve is configured to close when either the third comparison or thesecond comparison are satisfied.

In addition to one or more of the features described above, or as analternative, further embodiments of the refrigeration unit may includethat the controller is configured to monitor a second characteristic,wherein the second characteristic is at least one of (i) a time that aflash tank pressure exceeds a critical pressure modified by a firstpressure constant and (ii) the flash tank pressure

In addition to one or more of the features described above, or as analternative, further embodiments of the refrigeration unit may includethat the third comparison is satisfied if the time that the flash tankpressure exceeds the critical pressure modified by the first pressureconstant exceeds a predetermined time.

In addition to one or more of the features described above, or as analternative, further embodiments of the refrigeration unit may includethat the fourth comparison is satisfied if the flash tank pressureexceeds the critical pressure modified by a second pressure constant.

In addition to one or more of the features described above, or as analternative, further embodiments of the refrigeration unit may includethat the controller is further configured to determine if the economizersolenoid valve should be opened again after the economizer solenoidvalve is closed.

Technical effects of embodiments of the present disclosure include amethod of controlling a refrigeration unit to improve use of aneconomized mode of the refrigeration unit. Further technical effectsinclude a refrigeration unit configured to improve use of an economizedmode of the refrigeration unit. Further technical effects includecalculating a compressor middle stage pressure and using such calculatedcompressor middle stage pressure to determine when to operate arefrigeration unit in an economized mode.

The foregoing features and elements may be combined in variouscombinations without exclusivity, unless expressly indicated otherwise.These features and elements as well as the operation thereof will becomemore apparent in light of the following description and the accompanyingdrawings. It should be understood, however, that the followingdescription and drawings are intended to be illustrative and explanatoryin nature and non-limiting.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter is particularly pointed out and distinctly claimed atthe conclusion of the specification. The foregoing and other features,and advantages of the present disclosure are apparent from the followingdetailed description taken in conjunction with the accompanying drawingsin which:

FIG. 1A is a schematic view of an exemplary embodiment of a tractortrailer system having a refrigeration unit and a cargo compartment;

FIG. 1B is a schematic view of an exemplary embodiment of arefrigeration unit for a cargo compartment of the tractor trailer systemof FIG. 1A;

FIG. 2 is a schematic illustration of a refrigeration unit in accordancewith an embodiment of the present disclosure; and

FIG. 3 is a flow process of a method of operating a refrigeration unitin accordance with a non-limiting embodiment of the present disclosure.

DETAILED DESCRIPTION

As shown and described herein, various features of the disclosure willbe presented. Various embodiments may have the same or similar featuresand thus the same or similar features may be labeled with the samereference numeral, but preceded by a different first number indicatingthe figure to which the feature is shown. Thus, for example, element “a”that is shown in FIG. X may be labeled “Xa” and a similar feature inFIG. Z may be labeled “Za.” Although similar reference numbers may beused in a generic sense, various embodiments will be described andvarious features may include changes, alterations, modifications, etc.as will be appreciated by those of skill in the art, whether explicitlydescribed or otherwise would be appreciated by those of skill in theart.

Shown in FIG. 1A is a schematic of an embodiment of a tractor trailersystem 100. The tractor trailer system 100 includes a tractor 102including an operator's compartment or cab 104 and also including anengine, which acts as the drive system of the tractor trailer system100. A trailer 106 is coupled to the tractor 102. The trailer 106 is arefrigerated trailer 106 and includes a top wall 108, a directly opposedbottom wall 110, opposed side walls 112, and a front wall 114, with thefront wall 114 being closest to the tractor 102. The trailer 106 furtherincludes a door or doors (not shown) at a rear wall 116, opposite thefront wall 114. The walls of the trailer 106 define a cargo compartment.The trailer 106 is configured to maintain a cargo 118 located inside thecargo compartment at a selected temperature through the use of arefrigeration unit 120 located on or next to the trailer 106. Therefrigeration unit 120, as shown in FIG. 1A, is located at or attachedto the front wall 114.

Referring now to FIG. 1B, the refrigeration unit 120 is shown in moredetail. The refrigeration unit 120 includes a compressor 122, acondenser 124, an expansion valve 126, an evaporator 128, and anevaporator fan 130. The compressor 122 is operably connected to arefrigeration engine 132 which drives the compressor 122. Therefrigeration engine 132 is connected to the compressor in one ofseveral ways, such as a direct shaft drive, a belt drive, one or moreclutches, and/or via an electrical generator. A refrigerant line 123fluidly connects the components of the refrigeration unit 120.

Airflow is circulated into and through the cargo compartment of thetrailer 106 by means of the refrigeration unit 120. A return airflow 134flows into the refrigeration unit 120 from the cargo compartment of thetrailer 106 through a refrigeration unit inlet 136, and across theevaporator 128 via the evaporator fan 130, thus cooling the returnairflow 134 to a selected or predetermined temperature. The cooledreturn airflow 134, now referred to as supply airflow 138, is suppliedinto the cargo compartment of the trailer 106 through a refrigerationunit outlet 140, which in some embodiments is located near the top wall108 of the trailer 106. The supply airflow 138 cools the cargo 118 inthe cargo compartment of the trailer 106. It is to be appreciated thatthe refrigeration unit 120 can further be operated in reverse to warmthe trailer 106 when, for example, the outside temperature is very low.

The refrigeration unit 120 is positioned in a frame 142 and contained inan accessible housing 144, with the frame 142 and/or the housing 144secured to an exterior side of the front wall 114 such that therefrigeration unit 120 is positioned between the front wall 114 and thetractor 102, as shown in FIG. 1A.

It will be appreciated by those of skill in the art that the systems andconfigurations of FIGS. 1A and 1B are merely exemplary and provided forillustrative and descriptive purposes only. The invention is not limitedthereby. For example, although a tractor trailer configuration is shown,systems may be employed in other trailer configurations, in varioustruck configurations, and/or in other systems and configurations.

Turning now to FIG. 2, a refrigeration unit 220 in accordance with anon-limiting embodiment of the present disclosure is shown. Therefrigeration unit 220 includes a compressor 222 that may be a scrollcompressor; however, other compressors such as reciprocating or screwcompressors are possible without limiting the scope of the disclosure. Amotor (not shown) can be used to drive the compressor 222. For example,a motor can be an integrated electric drive motor driven by asynchronous generator, a commercial power service, an external powergeneration system (e.g., shipboard), a generator, or the like. Thecompressor 222 can be a multi-stage compression device.

In operation, high temperature, high pressure refrigerant vapor exitingthe compressor 222 is directed to a condenser 224, which can include aplurality of condenser coil fins and tubes, which receive air, typicallyblown by a condenser fan. As shown, the condenser 224 may include anintercooler 223 and a gas cooler 225. By removing heat through thecondenser 224, the refrigerant changes to a high pressure/lowertemperature state.

The refrigerant may flow from the gas cooler 225 of the condenser 224 toa flash tank 246. A high pressure expansion device 248 may be locatedbetween the gas cooler 225 and the flash tank 246. The flash tank 246may be fluidly connected to an evaporator 228 and to a portion of thecompressor 222. An electronic evaporator expansion valve 250 may belocated between the flash tank 246 and the evaporator 228. Therefrigerant may then flow from the evaporator 228 back to the compressor222, to form a complete refrigerant circuit.

As shown and noted above, the compressor 222 may be a multi-stagecompressor. As shown in FIG. 2, the compressor 222 includes a firststage 252 and a second stage 254. The refrigeration unit 220 can includean economizer solenoid valve 258 located between the flash tank 246 andthe second stage 254 of the compressor 222. When the economizer solenoidvalve 258 is active, the economizer solenoid valve 258 can open to allowrefrigerant to pass through the flash tank 246 to enter the compressorsecond stage 254. The economizer solenoid valve 258 can be controlledresponsive to a pressure of the flash tank 246 and a calculated middlestage pressure as described herein. The calculated middle stage pressuremay be a pressure within an economizer line 256 that is upstream of thesecond stage 254 of the compressor 222. The economizer solenoid valve258 may be located on and control a fluid flow through the economizerline 256. As shown, an output from the intercooler 223 may be connectedto an output of the economizer solenoid valve 258.

Many of the points in the refrigeration unit 220 can be monitored andcontrolled by a controller 260 and various associated sensors (notshown) located throughout the refrigeration unit 220. The controller 260can include a microprocessor and an associated memory. The memory ofcontroller 260 can contain operator and/or owner preselected, desiredvalues for various operating parameters within the refrigeration unit220 including, but not limited to, temperature set points for variouslocations within the refrigeration unit 220, pressure limits, currentlimits, engine speed limits, and any variety of other desired operatingparameters or limits with the refrigeration unit 220. In one embodiment,the controller 260 can include a microprocessor board that contains amicroprocessor and memory, an input/output (I/O) board that can includean analog to digital converter. The I/O board can receive temperatureinputs and pressure inputs from various points in the system, i.e. fromthe various sensors, AC current inputs, DC current inputs, voltageinputs and humidity level inputs. In addition, the I/O board can includedrive circuits or field effect transistors (“FETs”) and relays toreceive signals or current from the controller 260 and in turn controlvarious external or peripheral devices in the refrigeration unit 220.

Among exemplary sensors and/or transducers monitored by controller 260can be a return air temperature sensor that inputs into the controller260 a variable resistor value according to the evaporator return airtemperature. An ambient air temperature sensor that can provide thecontroller 260 an ambient air temperature value (e.g., detected in frontof or upstream of the condenser 224). A compressor suction temperaturesensor that can input to the controller 260 a variable resistor valueaccording to a compressor suction temperature. A compressor dischargetemperature sensor that can detect a compressor discharge temperatureinside the compressor 222. An evaporator outlet temperature sensor andan evaporator outlet pressure transducer that can detect an outlettemperature value and an evaporator outlet pressure of evaporator 228. Acompressor suction pressure transducer can provide the controller 260 avariable voltage according to a compressor suction value of thecompressor 222. A compressor discharge pressure transducer that canprovide to the controller 260 a variable voltage according to acompressor discharge value of compressor 222. In addition, a directcurrent sensor and an alternating current sensor can detect currentdrawn by the compressor 222. A sensor may be configured to read apressure on the flash tank 246.

One function of the controller 260 may be to control, e.g., open and/orclose the economizer solenoid valve 258. The controller 260 may controlthe economizer solenoid valve 258 to ensure the refrigeration unit 220operates in an economized mode whenever possible in order to gainincreased capacity and efficiency from the refrigeration unit 220. Onesolution to ensure optimized economized mode operation is to applydedicated and specific switch logic to force the system into economizedmode by opening the economizer solenoid valve 258; however this solutionmay create the risk of injecting high density refrigerant into thecompressor 222, which may be detrimental to the components of thesystem.

In some refrigeration systems, two conditions may exist to enable thesystem to run in the economized mode. The two conditions are (i)compressor middle stage pressure should be below the critical pressureof the refrigerant, and (ii) the high pressure expansion device 248 andthe electronic evaporator expansion valve 250 are capable to maintainthe pressure in the flash tank 246 at a pressure equal to a pressure inthe economizer line 256. Based on these two conditions, embodimentsdescribed herein provide an economizer solenoid valve, such as aneconomized solenoid valve, control process. The control process includestaking measurement of various pressures within the system, calculatingor predicting a compressor middle stage pressure, and defining criteriato determine the economized solenoid valve to be open or closed—and thusoperate in the economized mode.

Turning now to FIG. 3, a flow process of controlling a refrigerationunit in accordance with a non-limiting embodiment of the presentdisclosure is shown. The flow process 300 may be employed by arefrigeration unit having a flash tank and in some embodiments isconfigured as shown and described above. The flow process 300 may be anoperating routine or other program or programming that may be executedby a processor or other processing device. The flow process 300incorporates readings or measurements of pressure at various positionsor locations within a refrigeration unit and further involvescalculations that may be processed on a processor or other device.

At the beginning of the flow process 300, an economizer solenoid valvemay be in a closed state at block 302. In operation, one or morereadings or measurements may be taken within the refrigeration unit atblock 304. For example, the pressure within a flash tank P_(ft), acompressor suction pressure P_(suc), and a pressure at a discharge portP_(dis) may be measured. The values may be conveyed to or read by aprocessor or other control device or controller.

The controller may then predict a pressure at a middle stage of thecompressor P_(mid) at block 306. The middle stage pressure P_(mid) maybe based on the compressor suction pressure P_(suc) and the dischargepressure P_(dis). The calculation may be based, in part, on the designand configuration of the refrigeration unit, and may in someembodiments, be based on the design and/or configuration of thecompressor. For example, in a non-limiting embodiment, given a number ofconstants and/or known parameters (e.g., k₁, k₂, k₃), the middle stagepressure P_(mid) may be calculated with the following equation:P_(mid)=k₁×(P_(suc)×P_(dis))^(k) ² +k₃.

Next, the controller may make comparisons of the variousknown/calculated values to determine if the refrigeration unit should beoperated in an economized mode at block 308. For example, the calculatedmiddle stage pressure P_(mid) may be compared to a known and/orpredetermined value that is configured to enable operation of therefrigeration unit in an economized mode. Further, the flash tankpressure P_(ft) may be compared to a known and/or predetermined value.

For example, as shown at block 308 of FIG. 3, the middle stage pressureP_(mid) may be compared to a value that is a critical pressure of therefrigerant P_(crit). Further, the pressure of the flash tank P_(ft) mayalso be compared to the critical pressure of the refrigerant P_(crit).In some embodiments, as shown, the middle stage pressure P_(mid)comparison may require the critical pressure of the refrigerant P_(crit)to be modified by a first pressure constant dP₁. Thus, in the exampleembodiment of FIG. 3, at block 308, the controller may make the followcomparisons: a first comparison C₁: P_(mid)<(P_(crit)−dP₁) and a secondcomparison C₂: P_(ft)<P_(crit). If it is determined that either thefirst comparison C₁ or the second comparison C₂ is not satisfied, theeconomizer solenoid valve will remain closed and the economized modewill not be entered. If this is the case, the process 300 will return tothe start 302 and blocks 304-308 will be cycled or repeated to monitorfor when both the first comparison C₁ and the second comparison C₂ aresatisfied.

As such, when it is determined that both the first comparison C₁ and thesecond comparison C₂ are satisfied, the flow process 300 will continueto block 310 and the economizer solenoid valve will be opened. As shown,the economizer solenoid valve is referred to as an economized solenoidvalve, but those of skill in the art will appreciate that othereconomizer solenoid valves may be used without departing from the scopeof the present disclosure.

Next, at block 312, if the economizer solenoid valve is open for aperiod wherein if the flash tank pressure P_(ft) is greater than apredetermined value, the economizer solenoid valve will be closed. Thus,at block 312, the controller will make the following comparisons: athird comparison C₃: t[P_(ft)>(P_(crit)−dP₁)]>t₁ and a fourth comparisonC₄: P_(ft)>(P_(crit)+dP₂).

In the third comparison C₃, the controller is tracking the time that theflash tank pressure P_(ft) is greater than a pressure valve based on thecritical pressure of the refrigerant P_(crit) and comparing it againstthe predetermined time t₁. In the fourth comparison C₄, the controlleris monitoring the flash tank pressure P_(ft) and ensuring that thepressure in the flash tank P_(ft) does not exceed a pressure value basedon the critical pressure of the refrigerant P_(crit) as modified with asecond pressure constant dP₂.

If it is determined that neither the third comparison C₃ nor the fourthcomparison C₄ is satisfied, the economized mode will be maintained andthe economizer solenoid valve will remain open. However, if it isdetermined that either of the third comparison C₃ or the fourthcomparison C₄ are satisfied, the process 300 will continue to block 314and the economizer solenoid valve will be closed. Once the economizersolenoid valve is closed and the economized mode is exited, the process300 will begin again at block 302 and the pressures P_(ft), P_(suc), andP_(dis) will be measured and the middle stage pressure P_(mid) will becalculated (blocks 304, 306) and the values will be compared in thefirst comparison C₁ and the second comparison C₂ at block 308.

Advantageously, embodiments described herein provide a refrigerationunit having an improved economized mode operation and/or use. Forexample, advantageously in accordance with various embodiments, animproved economized solenoid valve control is provided by introducing apredicted or calculated compressor middle stage pressure into criteriafor determining when to open or close an economizer solenoid valve. Theeconomizer solenoid valve control as provided herein allows the systemto operate in an economized mode when it should, and further eliminatesthe use of special switch logic. Further, advantageously, embodimentsdescribed herein avoid the risk of damaging the compressor of arefrigeration unit. Moreover, embodiments provided herein may ensurethat a refrigeration unit or system will operate with optimized capacityand/or efficiency.

While the present disclosure has been described in detail in connectionwith only a limited number of embodiments, it should be readilyunderstood that the present disclosure is not limited to such disclosedembodiments. Rather, the present disclosure can be modified toincorporate any number of variations, alterations, substitutions,combinations, sub-combinations, or equivalent arrangements notheretofore described, but which are commensurate with the scope of thepresent disclosure. Additionally, while various embodiments of thepresent disclosure have been described, it is to be understood thataspects of the present disclosure may include only some of the describedembodiments.

For example, although various embodiments described herein includespecific components and/or arrangements of components, other componentsand/or substitutions thereof may be employed without departing from thescope of the present disclosure. Further, although a process isdescribed herein with a specific order of steps, those of skill in theart will appreciate that the process may include additional and/oralternative steps and further the steps may be performed in any orderand/or simultaneously, without departing from the scope of the presentdisclosure.

Accordingly, the present disclosure is not to be seen as limited by theforegoing description, but is only limited by the scope of the appendedclaims.

What is claimed is:
 1. A method for operating a refrigeration unit, the method comprising: measuring a first characteristic of a refrigeration unit; calculating a compressor middle stage pressure based on the first measured characteristic; determining if a first comparison and a second comparison are satisfied based on the first measured characteristic and the calculated compressor middle stage pressure; and opening an economizer solenoid valve when the first comparison and the second comparison are satisfied.
 2. The method of claim 1, wherein the first characteristic comprises a plurality of characteristics.
 3. The method of claim 2, wherein the first characteristic comprises a flash tank pressure, a compressor suction pressure, and a discharge port pressure.
 4. The method of claim 3, wherein the compressor middle stage pressure is calculated based on the compressor suction pressure and the discharge port pressure as P_(mid)=k₁×(P_(suc)×P_(dis))^(k) ² +k₃, wherein k₁, k₂, k₃ are known parameters.
 5. The method of claim 1, wherein the first characteristic comprises a flash tank pressure, the first comparison comprises comparing the calculated compressor middle stage pressure with a critical pressure modified by a first pressure constant, and the second comparison comprises comparing the flash tank pressure with the critical pressure.
 6. The method of claim 1, further comprising: monitoring a second characteristic; determining if a third comparison or a fourth comparison are satisfied based on the second characteristic; and closing the economizer solenoid valve when either the third comparison or the second comparison are satisfied.
 7. The method of claim 6, wherein the second characteristic is at least one of (i) a time that a flash tank pressure exceeds a critical pressure modified by a first pressure constant and (ii) the flash tank pressure.
 8. The method of claim 7, wherein the third comparison is satisfied if the time that the flash tank pressure exceeds the critical pressure modified by the first pressure constant exceeds a predetermined time.
 9. The method of claim 7, wherein the fourth comparison is satisfied if the flash tank pressure exceeds the critical pressure modified by a second pressure constant.
 10. The method of claim 6, further comprising, after closing the economizer solenoid valve, determining if the economizer solenoid valve should be opened again.
 11. A refrigeration unit comprising: at least one sensor configured to measure a first characteristic of the refrigeration unit; a controller configured to calculate a compressor middle stage pressure based on the first characteristic the controller further configured to determine if a first comparison and a second comparison is satisfied based on the first characteristic and the calculated compressor middle stage pressure; and an economizer solenoid valve configured to open when the first comparison and the second comparison are satisfied.
 12. The refrigeration unit of claim 11, wherein the first characteristic comprises a plurality of characteristics.
 13. The refrigeration unit of claim 12, further comprising a flash tank, wherein the first characteristic comprises a flash tank pressure, a compressor suction pressure, and a discharge port pressure.
 14. The refrigeration unit of claim 13, wherein the compressor middle stage pressure is calculated based on the compressor suction pressure and the discharge port pressure as P_(mid)=k₁×(P_(suc)×P_(dis))^(k) ² +k₃, wherein k₁, k₂, k₃ are known parameters.
 15. The refrigeration unit of claim 11, further comprising a flash tank, wherein the first characteristic comprises a flash tank pressure, the first comparison comprises comparing the calculated compressor middle stage pressure with a critical pressure modified by a first pressure constant, and the second comparison comprises comparing the flash tank pressure with the critical pressure.
 16. The refrigeration unit of claim 11, wherein the controller is configured to determine if a third comparison or a fourth comparison are satisfied and wherein the economizer solenoid valve is configured to close when either the third comparison or the second comparison are satisfied.
 17. The refrigeration unit of claim 16, wherein the controller is configured to monitor a second characteristic, wherein the second characteristic is at least one of (i) a time that a flash tank pressure exceeds a critical pressure modified by a first pressure constant and (ii) the flash tank pressure
 18. The refrigeration unit of claim 17, wherein the third comparison is satisfied if the time that the flash tank pressure exceeds the critical pressure modified by the first pressure constant exceeds a predetermined time.
 19. The refrigeration unit of claim 17, wherein the fourth comparison is satisfied if the flash tank pressure exceeds the critical pressure modified by a second pressure constant.
 20. The refrigeration unit of claim 16, wherein the controller is further configured to determine if the economizer solenoid valve should be opened again after the economizer solenoid valve is closed. 